The present invention relates generally to the field of fossil fuel cyclone-fired boilers, and in particular to the selective use of oxygen enrichment at strategic points in the barrel of a slagging cyclone combustor to maintain desired slag flow characteristics, thereby broadening the range of amenable fuels and operating conditions while lowering operating costs, improving combustion efficiency, and reducing nitrogen oxide emissions.
Cyclone-fired boilers were developed in the 1940s primarily to improve the firing of coals with low ash-fusion temperature through minimizing the ash-induced slagging/fouling of high temperature heat transfer surfaces within the boiler. This was accomplished by combusting the coal and simultaneously melting the ash in a high-temperature chamber adjacent to the boiler, discharging the essentially ash-free products of combustion into the boiler and draining the molten slag to a tank at the bottom of the furnace. While this indeed reduced boiler-side fouling, the need to maintain a continuously-draining molten ash placed restrictions on the coal supply that generally increase the cost of powering the unit. Moreover, localized slag solidification within the cyclone is known to reduce unit availability while also hampering the ability to lower the firing rate and alter the stoichiometric ratio of the combustion process. (The stoichiometric ratio represents the relative proportion of oxidant to fuel used in the combustion process. A stoichiometric ratio of 1.0 is the theoretical minimum needed for complete combustion of the fuel, while a stoichiometric ratio less than 1.0 signifies fuel-rich combustion.) Finally, because of the high-temperatures needed to melt the slag, and the tendency to run at or above stoichiometric conditions, cyclone combustors are known to generate relatively high levels of nitrogen oxide (NOx) emissions, typically in the range of 1.0-2.0 lb NO2/MMBtu prior to post-combustion treatment. Hence, the limitations on NOx imposed by the 1990 Clean Air Act Amendments are particularly challenging and costly to achieve within fossil fuel cyclone-fired boilers.
A typical cyclone combustor 10 is illustrated in FIG. 1. Conventional combustion within a slagging cyclone combustor of this design is carried out by injecting crushed coal and primary air through a coal pipe 12 to a burner 14. Tertiary air enters the burner at the tertiary air inlet 16, and secondary air (the main combustion air) enters the cyclone combustor at the secondary air inlet 18. The burner, which imparts a swirl motion to the crushed coal in the same rotation as the secondary air, injects a coal/air mixture with high tangential velocity into a refractory lined combustion chamber or barrel 20. The coal is crushed (˜95% through 4 mesh [4.8 mm] screen) rather than pulverized (˜70-80% through 200 mesh [70 micron] screen) to minimize the escape of fines from the barrel. Coal particles are thrown outward as the flow spins through the barrel, creating a region of high heat release adjacent the refractory lining of the barrel wall. The high temperature in this region causes the ash contained within the coal to melt. The molten “slag” 22 acts as a trap for the carbon-rich coal particles, retaining the particles for a period of time far greater than the average gas residence time within the barrel, thereby enabling a high degree of carbon burnout. The molten slag eventually migrates forward along the wall of the barrel, exits at the slag spout opening 24, and continuously drains through a slag tap opening 26 located below the re-entrant throat 28. The gas flow makes a triple pass—initially swirling along the barrel wall toward the re-entrant throat, then swirling upstream within an annular region, and finally turning and exiting from the barrel through the re-entrant throat into the furnace 30.
Oxygen enrichment has not been used in cyclone combustors to refuel cyclone-fired boilers with coals that are not amenable to air-fuel slagging operation. However, the use of oxygen enrichment to maintain molten slag and accelerate combustion within cyclone combustors has been considered. For example, U.S. Pat. No. 2,745,732 (Oster) discloses the use of oxygen enrichment in a cyclone combustor to sustain a molten slag layer under reducing conditions in order to maximize recovery of metallic iron from the ash. The oxygen used for this purpose is introduced via a high-velocity, pre-heated, oxygen-enriched secondary air stream injected tangentially into the cyclone.
U.S. Pat. No. 4,598,652 (Hepworth) discusses the possibility of using oxygen enrichment in a coal-fired cyclone combustor in which iron oxide particles are injected for sulfur capture. Although oxygen enrichment is mentioned as a possible means of accelerating the rates of reaction, there is no discussion regarding how or where the oxygen would be introduced into the cyclone combustor.
Techniques for controlling ash viscosity in cyclones without the use of oxygen enrichment also have been considered. U.S. Pat. No. 5,022,329 (Rackley, et al.) and U.S. Pat. No. 5,052,312 (Rackley et al.) teach the addition of fluxing agents to maintain the T250 of the ash below 2500° F. to limit the vaporization of heavy metals. (The T250 value denotes the temperature at which a coal slag has a viscosity of 250 centipoise.)
U.S. Pat. No. 6,085,674 (Ashworth) discusses the addition of lime or limestone into a cyclone to lower ash viscosity. U.S. Patent Application No. 2002/0184817 (Johnson, et al.) describes the use of an iron-based additive to modify the viscosity and slagging characteristics of coals, particularly low-sulfur Western U.S. coals.
With regard to NOx reduction in cyclone combustors, U.S. Pat. No. 5,878,700 (Farzan, et al.) proposes injection of a secondary fuel (reburn fuel) along the axis of the barrel to convert NOx formed within the barrel to N2 as gases are discharged from the unit. U.S. Pat. No. 6,085,674 (Ashworth) proposes NOx reduction through a combination of steam injection and a three-stage combustion process comprised of a fuel-rich barrel operation followed by two distinct stages of air addition. U.S. Pat. No. 6,325,002 (Ashworth) further proposes injection of tertiary and overfire air in such a way as to create in-situ recirculation of flue gases to dilute the products of combustion and further lower NOx. None of these references discloses or teaches the use of oxygen enrichment as a means to reduce NOx.
Several references contemplate NOx reduction with the aid of oxygen enrichment, but without specific reference to cyclone combustors. U.S. Pat. No. 4,427,362 (Dykema) describes a combustion method requiring a high-temperature, fuel-rich first stage for the purpose of reducing NOx emissions. The high temperature (at least 1800K) is required to accelerate reaction kinetics, while fuel-rich conditions (stoichiometric ratio between 0.45-0.75) are needed to establish equilibrium chemistry with minimal NOx formation. Although this patent mentions the possibility of using oxygen enrichment, it does not provide any information on how oxygen would be introduced into the system. Moreover, this patent does not teach the use of oxygen enrichment in cyclone combustors.
A similar approach to NOx reduction is discussed in U.S. Pat. No. 4,343,606 (Blair, et al.) except that this reference includes one or more secondary air injection points to complete combustion, while also omitting particulate injection. This patent teaches a first stage equivalence ratio of greater than about 1.4 (stoichiometric ratio less than about 0.7), while allowing for enrichment of air with between 6 and 15 weight percent oxygen. However, no details are provided regarding the means of introduction of the oxygen, nor is there any discussion regarding operational issues specific to cyclone combustors.
U.S. Pat. No. 6,394,790 (Kobayashi) discloses a method for NOx reduction via deeply staged (i.e., exceedingly fuel-rich) oxygen-enriched primary combustion coupled with secondary oxidant injection. The oxygen concentration of the primary oxidizer is at least 30%, while the required oxidizer to fuel ratio in the primary stage is between 5% and 50% of stoichiometric. This patent teaches that high velocity injection of reactants is key to NOx reduction since the vigorous mixing it induces will serve to lower the reaction temperature. The only solid fuel explicitly mentioned in this patent is pulverized coal, suggesting that application to slagging cyclone combustors was not intended.
U.S. Patent Application No. 2003/0009932 (Kobayashi, et al.) also addresses NOx reduction in coal-fired boilers via a fuel-rich first combustion stage with oxygen enrichment up to 8 volume percent. No fixed limits are placed on the first stage stoichiometric ratio, and no mention is made of ash fusibility or viscosity. Several references are made to pulverized coal (in contrast to crushed coal) and low NOx burners, suggesting that application of the method to cyclone combustors was not intended. This patent application suggests that there is a certain stoichiometric ratio (not precisely quantified) below which NOx emissions will be reduced with oxygen-enriched combustion relative to air-fuel combustion. However, the application does not contemplate the influence of aerodynamics, mixing or particle time-temperature history on NOx characteristics.
A fundamental requirement for stable operation of a slagging cyclone combustor is that the ash layer remains in a molten state with sufficiently low viscosity to permit adequate drainage of the slag. Difficulties in achieving this condition contribute to reduced on-stream time and restricted load-following capability in conveniently-operated air-fuel slagging cyclone combustors. Experience has determined that the critical viscosity for adequate drainage is 250 centipoise. As previously noted, the temperature corresponding to this viscosity level is T250. Stable operation of a slagging cyclone combustor requires the temperature of the slag to be greater than or equal to T250. This requirement places limits on the allowable range of coals and operating conditions, while also contributing to higher NOx emissions than encountered in many pulverized coal combustion systems.
It is desired to have a method and a system to permit refueling of cyclone combustors with coals that are not amenable to air/fuel-fired cyclone operation due to the inability to maintain a molten slag layer of sufficiently low viscosity to permit continuous slag flow.
It is still further desired to have a method and a system to minimize the escape of fine coal particles from the barrel of a cyclone.
It is still further desired to have a method and a system to lower NOx emissions in slagging cyclone combustors, primarily (but not exclusively) by broadening the ranges of stoichiometric ratio and firing rate, relative to air-fuel operation, under which a molten slag layer can be maintained.
It is still further desired to have a method and a system to improve unit availability (i.e., on-stream time) by minimizing temperature excursions that result in freezing of the slag.
It also is desired to have a method and a system for combusting a fuel in a cyclone combustor which afford better performance than the prior art, and which also overcome many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.